by Tony » Mon May 07, 2007 8:02 pm
I still prefer a single tube plus reservoir to a U tube, because the scale is much easier to read. But I do own a fairly large mercury U tube manometer that I use as a pressure calibration reference for checking pressure gauges against. I just use a loose steel rule to measure directly between the two mercury levels. It works but is not very convenient.
With a single tube, 28" pressure goes 28" up the scale. I could not be simpler. And a store bought ruler will probably look better than a scale you have to make yourself.
With a U tube 28" travels only 14" up one side, and 14" down the other. So resolution is really only half as good. That is if you are only measuring on one side of the U from zero, which is the most usual way to do it. Zeroing a U tube can be quite an entertaining process.
As 2seater says, one of these single tube manometers can measure either positive or negative pressure, and for a single direction flow bench, the connections to the bench and manometer need to be swapped around, which only takes about five seconds to do.
These single tube manometers are so simple to construct, why not fit two to the bench ? one for the blow side test pressure, and one for suck side test pressure. Then nothing need be changed around.
I did not think to do this, but in retrospect now wish that I had made a pair of identical matching test pressure manometers instead of only one.
Also known as the infamous "Warpspeed" on some other Forums.